Downhole logging systems are used to measure the physical, chemical, and structural properties of subterranean geological formations. The downhole logging systems generally include a downhole tool that is either lowered into a borehole on a wireline cable (referred to as “wireline logging”) or placed just behind a drill bit as part of the drill pipe itself (referred to as “logging-while-drilling”). The logging systems may employ various acoustic, nuclear, and electrical measurement techniques to acquire downhole logs of the properties of a downhole formation.
One type of downhole logging system makes use of a physical phenomenon known as Compton scattering. Generally, in Compton scattering, the number of backscattered photons from an object in front of a photon source is a function of photon energy and backscattering angle. The material properties, such as the attenuation coefficient, thickness and density of the object may also affect the number of backscattered photons. Downhole tools may be designed to emit photons with a photon energy in a region where Compton scattering is dominant. These photons are then backscattered by materials in the borehole and subsequently detected by equipment on the tool. The backscattering angle of the photons that the tool is sensitive to may be determined by the design of the detector collimator geometry. The number of backscattered and detected photons may be proportional to the density of the object, assuming single Compton scattering occurs. Thus, the tool can be used to determine the material density of an object in proximity to the logging system by counting the number of photons backscattered from the object. However, due to multiple scattering of photons and other shortcomings in conventional downhole tools, a need exists in the art for downhole tools with a high azimuthal and radial sensitivity to backscattered photons.